5.10 Additional Floating Types

As an extension, the GNU C compiler supports additional floating
types, __float80 and __float128 to support 80bit
(XFmode) and 128 bit (TFmode) floating types.
Support for additional types includes the arithmetic operators:
add, subtract, multiply, divide; unary arithmetic operators;
relational operators; equality operators; and conversions to and from
integer and other floating types. Use a suffix `w' or `W'
in a literal constant of type __float80 and `q' or `Q'
for _float128. You can declare complex types using the
corresponding internal complex type, XCmode for __float80
type and TCmode for __float128 type: